If, for whatever reasons, an employer wants to know, what sort of a person you are with your friends — and they all will, once the positions they are considering you for reach a certain height, they'll find out. With private investigators, if need be.
And that makes it OK?
What you present to the employer being separate from your personal life is actually a really important part of how we function as a society.
Is it? How so? Can you cite any studies showing usefulness of such separation? Or how this separation changed over the years — for the betterment of society, or otherwise?
Well clearly I'm not going to have such studies to hand, not sure how you would study such a thing, but this one touches on similar subjects showing how there is inbuilt racism / nationalism in CV selection. That sort of problem is only going to get worse with the more information available.